Feature-flag test matrix design: the flag-state combinatorics problem (N flags × M variants × K user-segments = N×M×K test cases), the canonical coverage strategies (pairwise interaction coverage; default-only smoke; full matrix; risk-driven matrix), the workflow for building the coverage suite from a flag inventory (grep-based inventory, per-flag classification, PICT pairwise generation, per-cell test skeletons), the dedicated kill-switch test categories (references/killswitch.md: graceful degradation, fail-static default, kill latency, mid-flight consistency), and the flags-vs-experiments distinction. Use when designing the flag-test surface for a new project, building or auditing flag-test coverage, or authoring kill-switch tests.